Photo-supercapacitors based on nanoscaled ZnO

In this study, zinc oxide (ZnO) powders in two different morphologies, nanowire (NW) and nanoflower (NF), have been synthesized by the hydrothermal method. The eligibility of the pristine ZnO nanopowders as a photo-active material has been revealed by designing P-SC devices via the facile drop-casting method on both glass and plastic substrates in large-area applications. The impact of physical properties and especially defect structures on photo-supercapacitor (P-SC) performance have been explored. Although the dark Coulombic efficiency (CE%) of both NW and NF-based P-SC were very close to each other, the CE% of NW P-SC increased 3 times, while the CE% of NF P-SC increased 1.7 times under the UV-light. This is because the charge carriers produced under light excitation, extend the discharge time, and as confirmed by electron paramagnetic resonance, photoluminescence, and transmission electron microscopy analyses, the performance of P-SCs made from NF powders was relatively low compared to those produced from NW due to the high core defects in NF powders. The energy density of 78.1 mWh kg−1 obtained for NF-based P-SCs is very promising, and the capacitance retention value of almost 100% for 3000 cycles showed that the P-SCs produced from these materials were entirely stable. Compared to the literature, the P-SCs we propose in this study are essential for new generation energy storage systems, thanks to their ease of design, adaptability to mass production for large-area applications, and their ability to store more energy under illumination.


Device Fabrication and Characterizations
The synthesized nanoparticles (0.1 g) have been dispersed in an isopropanol/1butanol/methanol mixture (3/1/0.5 volume ratio) in an ultrasonic bath for 10 min. The dispersion has been spun on FTO at 1000rpm / 30 s three times, followed by drop-casting to achieve the desired amount of active material on the surface. ZnO nanoparticles have assembled the P-SCs coated electrode (1.5 cm x 1.5 cm), and bare FTO coated glass substrates with filter paper sandwiched in between as a separator. PVA / LiCl gel was used as a solid electrolyte. According to a previously reported procedure, PVA / LiCl gel electrolyte has been prepared. [1] Briefly, 3 g PVA was mixed with 30 mL LiCl (5 M) aqueous solution and heated at 85 °C for 1 h under vigorous stirring. The electrodes and the separator were dipped in the PVA/LiCl solution, solidified, and kept at room temperature overnight. Flexible P-SC has been deposited on indiumdoped tin oxide (ITO) coated PET substrates applying the same procedure described above. Energy Dispersive X-ray (EDX) analyzer. Photoluminescence (PL) of ZnO NW and NF was generated by a HeCd laser (325 nm) and the emission spectra were recorded in the range 350-1100 nm by Ocean Optics Spectrometer QE65. The time-resolved PL (TR-PL) tests for DLE emission were performed using an NL100 nitrogen laser (Stanford research) as an excitation source. The excitation wavelength is 337 nm, and the pulse duration is 3.5 ns. The pulse energy is 170 µJ, which results in a peak power of 45 kW and average power of 3 mW. As a synchronized detector, a multi-alkali amplified PMT (Thorlabs) operating in the range 230 -920 nm was used. Figure 1F shows TR-PL spectra indicating that the PL decay of ZnO NF is longer than that of NW powder.

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The X and Q-band Electron Paramagnetic Resonance (EPR) spectroscopy measurements were carried out with a continuous-wave Elexsys 500 EPR spectrometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with a Bruker X-SHQ 4119HS-W1 X-Band resonator and a WT-Q. The UV irradiation of the samples for the EPR measurements was carried out with an M365FP1 -365 nm Fiber-Coupled LED from Thorlabs. Optical properties have been investigated using a PerkinElmer UV − vis spectrometer Lambda2S (200 − 1100 nm). The Electrochemical Impedance Spectroscopy (EIS) measurements have been performed at the potential of 0V over the frequency range from 1MHz to 0.1 Hz using a Gamry workstation under dark and UV light. The performance evaluations of P-SC have been investigated by electrochemical characterization methods such as Cyclic Voltammetry (CV), and galvanostatic charge/discharge (GCD) in a two-electrode configuration system by assembling the supercapacitor electrodes in Gamry potentiostat/galvanostat workstation under dark and UV illumination. A UV radiation source (8.8 mW.cm -2 ) has been used for the study of UV-irradiation-dependent electrochemical measurements. Coulombic efficiency (%), specific capacitance (Cp, F.g -1 ), specific energy (E, Wh.kg -1 ), and specific power (P, W.kg -1 ) of the supercapacitor device have been evaluated from the equations given in the supporting information.

Calculation of GCD and CV performance
Specific capacitance (C) has been calculated from CV data using the following equation [2] .
where C (F.g −1 ) denotes the specific capacitance, while V (V) corresponds to the potential window, I (A) represents the discharge current, and m (g) is the mass of the active material, and k (V.s −1 ) is scan rate.

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The discharge section of the GCD curves could be applied to evaluate Cp from Equation Moreover, by the Cs values, E and P of the supercapacitor cell can be evaluated as; [4] where Cs is the specific capacitance of the electrode, td is time (s) of discharge, tc is time (s) of charging, I is the discharge constant current (A), m is the mass of material (g), and ΔV is the potential window (V).   Flexible ZnO NF@Dark S12